Communication systems, such as the Profibus, as is described in DIN Standards 19245 Part 1 and 19245 Part 2 and in the document by Peter Neumann: Kommunikationssysteme in der Automatisierungstechnik, [Communication systems in automation engineering], Verlag Technik, Berlin, are used to provide the information interchange between various subsystems in a technical installation.
One special feature in this context is real-time data interchange as is required, for example, for process monitoring or for automatic control of technical systems. Profibus (Process Field Bus) is a standardization method that is used by automation companies and scientific organizations in the Federal Republic of Germany.
Profibus is a fieldbus system and is thus used, in the narrow sense, for standardized connection of automation devices in the field area.
The areas in which Profibus is used include, for example, autonomous use as a process data bus system for automation projects or as a subordinate communication level in process control systems. The communication or bus subscribers may be programmable logic controllers or control systems, numeric controllers or control systems, a local automation device or sensors and/or actuators with bus interfaces.
Each bus subscriber is a terminal of the communication system. As such, each subscriber has a communication means which allows the subscriber to be connected to the communication system, the bus. The communication means, which may have its own processor or a so-called ASIC, controls the transmission processes independently of the other subscriber actions.
The data interchange between the communication means and the terminal takes place, for example, by virtue of the fact that part of the memory in the terminal is mapped, virtually as a xe2x80x9cwindowxe2x80x9d across to the memory provided in the communication means. This method is called shared memory or memory mapped I/O. Data from the bus are buffered in the storage area provided for this purpose in the communication means. The terminal""s control unit can then read the data directly from the communication means as if said data were already stored in its own main memory.
For Profibus, a distinction is drawn between a physical topology and a logic topology. The physical topology is defined by a line interface in accordance with RS-485. Thus, the Profibus is a serial bus. In contrast, the logic topology corresponds on the one hand, to specific bus subscribers being linked in a ring, these subscribers being called active subscribers in the following text, and on the other hand, to in each case one active subscriber being linked in the form of a star to the other bus subscribers which are not included in this linking in the form of a ring, and which are called passive subscribers in the following text.
The authorization for bus access among the active bus subscribers is allocated in accordance with an identification method, the most common method being the token ring method. The token, a specific bit pattern which acts as an identification, is passed on from one active subscriber to the next active subscriber corresponding to the logic linking in the form of a ring. The active subscriber which has the token internally has access authorization to the medium, this authorization allowing access to the bus either for a limited time or for a fixed number of transmission processes.
Logic linking in the form of a star exists between, in each case, one active subscriber and one or more passive subscribers. The active subscriber in such a link in the form of a star is called the master in the following text. Data is retrieved from the passive bus subscribers by means of a global function which is implemented in the respective active subscriber. This retrieval is called polling in the following text. A passive subscriber may not access the medium (transmit data) until it is granted xe2x80x9cpermissionxe2x80x9d (polling) by the respective active subscriber.
A disadvantage of the conventional communication method is that it is impossible for passive communication subscribers which do not have their own transmission authorization to interchange data directly.
An, the object of the present invention is to provide a passive communication subscriber which has the capability to receive directly a data item transmission via the bus from another passive communication subscriber.
This object is achieved by providing a listening communication subscriber (S32-S34) for connection to a bus system (B) for industrial control systems, in particular a bus system (B) with programmable logic controllers, in which
a data item which is to be transmitted via the bus having at least one destination identifier and one source identifier,
the listening communication subscriber having its own identifier,
a filter table having at least one entry in each case having at least two parameters being provided for the listening communication subscriber,
the first parameter representing a destination identifier,
the second parameter representing a source identifier,
the listening communication subscriber in each case receiving a data item on the one hand when the destination identifier of the data item matches with its own identifier and
the listening communication subscriber in each case, on the other hand, receiving a data item when the destination identifier and the source identifier of the data item match the corresponding parameters of an entry in the filter table.
According to an advantageous refinement, the listening communication subscriber is provided with a buffer storage area in which a received data item can be buffered and from which the data item can be transferred to an input storage area as soon as it is found that the received data item is intended for the respective listening communication subscriber. This allows the data transmitted via the bus to be stored temporarily with the capability to evaluate the destination and source identifiers of the received data item within the buffer storage area, so that only one data item, which is actually intended for the respective listening communication subscriber, is transferred to its input memory area so that, the processing and evaluation of the wanted data in the received data item are not influenced by the configuration according to the invention of the listening communication subscriber.
A further object of the present invention is to provide a communication system in which a data item which has been transmitted via the bus by a first passive communication subscriber can be received directly by another passive communication subscriber.
The object is achieved in that, in a communication system which is formed by a bus system for industrial control systems, in particular a bus system having programmable logic controllers, and has active communication subscribers and passive communication subscribers, at least one of the passive communication subscribers is a listening communication subscriber,
a data item which is to be transmitted via the bus having at least one destination identifier and one source identifier,
the listening communication subscriber (S32-S34) having its own identifier (K),
a filter table having at least one entry in each case having at least two parameters being provided for the listening subscriber,
the first parameter representing a destination identifier,
the second parameter representing a source identifier,
the listening communication subscriber in each case receiving a data item on the one hand when the destination identifier of the data item matches with its own identifier, and
the listening communication subscriber in each case, on the other hand, receiving a data item when the destination identifier and the source identifier of the data item match the corresponding parameters of an entry in the filter table.
In a communication system having active communication subscribers and passive communication subscribers and which is formed by a bus system for industrial control systems, in particular a bus system having programmable logic controllers, the listening communication subscriber according to the present invention can be used advantageously since a listening communication subscriber designed according to the invention can handle the communication processes within the communication system more effectively.
In an advantageous refinement of the communication system, the listening communication subscriber is distinguished by the fact that it is provided with a buffer storage area in which a received data item can be buffered and from which the data item can be transferred to an input storage area as soon as it is found that the received data item is intended for the respective listening communication subscriber. The advantages for the individual communication subscriber configured according to the present invention also apply analogously to the communication system in which the communication subscriber is used.
A further object of the present invention is to provide a communication method which allows at least direct communication between passive communication subscribers.
This object is achieved by providing a communication method for a listening communication subscriber (S32-S34) for connection to a bus system (B) for industrial control systems, in particular a bus system (B) with programmable logic controllers, a data item (D) which is to be transmitted via the bus (B) having at least one destination identifier (T) and one source identifier (S), the listening communication subscriber (S32-S34) having its own identifier (K), and a filter table (F) having at least one entry in each case having at least two parameters (FTx, FSx) being provided for the listening communication subscriber (S32-S34), the first parameter (FTx) representing a destination identifier (T) and the second parameter (FSx) representing a source identifier (S), and the method comprising at least the following steps:
the listening communication subscriber (S32-S34) in each case receives a data item (D) on the one hand when the destination identifier (T) of the data item (D) matches its own identifier (K), and
the listening communication subscriber (S32-S34) in each case receives a data item (D) on the other hand when the destination identifier (T) and the source identifier (S) of the data item (D) match the corresponding parameters (FTx, FSx) of an entry in the filter table (F).
In an advantageous refinement of the communication method, a buffer storage area is provided in which a received data item is buffered and from which the data item is transferred to an input storage area as soon as it is found that the received data item is intended for the respective listening communication subscriber. The advantages for the individual communication subscriber configured according to the invention also apply analogously to the communication method.
The solutions and their refinements according to the present invention can advantageously be used with a serial bus, in particular the Profibus, as the bus system. A serial bus allows interference-free data transmission even over long distances and is thus particularly suitable for use in industrial environments. The Profibus has been proven for use in automation projects and, thus, is in widespread use.